[Enhancement] Improve naming consistency for Panda gripper finger attributes

[songyuc]

Description

I noticed an inconsistency between the URDF link names and the Python attribute names for the Panda robot's gripper fingers.

Current situation:

• URDF link names: panda_leftfinger, panda_rightfinger
• Python attributes: finger1_link, finger2_link

While the numeric indexing (1, 2) is functional, it creates a semantic gap where users need to remember which number corresponds to which finger (left or right).

Proposed Enhancement

I suggest adding more semantically clear attribute names that align with the URDF naming:

Option 1 (Recommended):

• Add new attributes: left_finger_link, right_finger_link
• Keep existing finger1_link, finger2_link as aliases for backward compatibility

Option 2 (Alternative):

• Use shorter names: lfinger_link, rfinger_link

Benefits

1. Improved readability: Code becomes self-documenting - no need to remember the finger1/finger2 mapping
2. Consistency: Aligns Python attribute names with URDF naming conventions
3. Better maintainability: Easier for new contributors and users to understand the code
4. Backward compatibility: Existing code continues to work if we keep aliases

Example

Current code:

l_contact_forces = self.scene.get_pairwise_contact_forces(
    self.finger1_link, object  # Which finger is this?
)

Proposed code:

l_contact_forces = self.scene.get_pairwise_contact_forces(
    self.left_finger_link, object  # Clear and intuitive
)

Implementation Suggestion

In panda.py, the _after_init method could be updated to:

def _after_init(self):
    # Use semantic names as primary
    self.left_finger_link = sapien_utils.get_obj_by_name(
        self.robot.get_links(), "panda_leftfinger"
    )
    self.right_finger_link = sapien_utils.get_obj_by_name(
        self.robot.get_links(), "panda_rightfinger"
    )
    
    # Keep backward compatibility
    self.finger1_link = self.left_finger_link
    self.finger2_link = self.right_finger_link
    
    # Similar updates for finger pads
    self.left_finger_pad_link = sapien_utils.get_obj_by_name(
        self.robot.get_links(), "panda_leftfinger_pad"
    )
    self.right_finger_pad_link = sapien_utils.get_obj_by_name(
        self.robot.get_links(), "panda_rightfinger_pad"
    )
    
    # Backward compatibility
    self.finger1pad_link = self.left_finger_pad_link
    self.finger2pad_link = self.right_finger_pad_link
    
    self.tcp = sapien_utils.get_obj_by_name(
        self.robot.get_links(), self.ee_link_name
    )

Context

I'm a graduate student studying the codebase. I noticed this naming pattern and thought it could be improved for better code clarity.

I'm happy to working on a PR for this enhancement if the maintainers agree with this proposal.

Thank you for maintaining this excellent robotics framework! 🚀